The present invention relates to composite structures and to methods and apparatuses pertaining to same, more particularly wherein the composite structures are to some degree or in some respect characterized by lamination.
Many composite structures include layers which are bonded together. Various applications have given rise to concern about delamination resistance at either or both of primary bond sites and secondary bond sites. The term "delamination resistance" is conventionally understood to encompass "strength" (e.g., through-thickness tensile strength) and/or "toughness" (e.g., Mode I fracture toughness). The terms "through-thickness strength," "out-of-plane strength" and "interlaminar strength" are synonymous in conventional usage.
Improvement of the delamination resistance of composite laminates has been attempted through a variety of mechanisms. Among the known mechanical methodologies for increasing delamination resistance are the following: (i) the insertion of metal pins, stitches or fibrous rods through the thickness of the composite laminate; and, (ii) the alteration of the style of reinforcement, e.g., through utilization of tufted fabrics to improve adhesion. There are drawbacks associated with these mechanical methodologies, such as cost, degradation of mechanical properties in the plane of the laminate, etc. Another conventional methodology for enhancing delamination resistance involves toughening of brittle resins with particles made of rubber (or another high elongation material); according to these approaches, toughness is generally achieved at the expense of strength.
It is often desirable to improve both strength and toughness, for the ability to do so could delay both crack initiation and crack propagation in composite laminates. Furthermore, any improvements in through-thickness strengths in composite laminates can be viewed as advantageous, since their low strengths in that direction are usually the limiting factor in design of structures with composites. Moreover, through-thickness strength is normally very sensitive to quality; thus, improvements in toughness could minimize the flaw sensitivity of the through-thickness strength. This is significant particularly because through-thickness stresses tend to arise in structural details which are difficult to fabricate at the level of quality of flat panels.
Composite structural details for U.S. Navy marine applications frequently require the use of secondary bonds for fabrication in a shipyard environment. Secondary bond sites are interfaces where there has been lamination over a cured laminate, and they can represent a weak link in composite laminate performance. The typical microstructural appearance of a secondary bond is a discrete, linear resin-rich region between the layers of a composite laminate. This resin-rich region can result in a composite laminate with reduced strengths through-the-thickness of the laminate (i.e., normal to the secondary bond) and reduced resistance to delamination.